In case of the use of gas warning means, it is necessary essentially for two reasons to let the gas sensors used respond in a specific manner. First, an undiagnosed failure of a gas sensor, e.g., due to blockage of the entry of gas or inactivation of the detector element, may lead to safety-relevant risks in an area being monitored. The sensor must therefore be subjected to function tests at short intervals. Second, commercially available gas sensors have a drift in the sensitivity of the sensor with respect to the species to be detected. This behavior of the sensor cannot be described or predicted by mathematical formulas. It is therefore necessary to calibrate sensors within certain time intervals with a target gas of a known concentration. The duration of the time intervals is determined by the requirements imposed on the desired accuracy of the sensor. Guidelines for the handling of this problem were summarized, e.g., in the specification T021 of the Trade Association of the Chemical Industry.
The proper function of the sensor is tested best by admitting the target gas into the sensor. This is the only way the entire functional chain from the gas supply to the signal generation can be checked. It is therefore common practice to stock calibrating agents, e.g., in the form of gas cylinders, often with toxic gases, to transport these to the sensor, and finally to introduce the test gas into the gas inlet of the particular sensor through suitable devices, e.g., pumps, valves and/or mass flow controllers. The expense of carrying out these function tests and calibration procedures is high.
To avoid this expense, it is known that a gas generator can be accommodated together with a gas sensor in a common housing (GB 2,254,696). The common housing is limited toward the measured gas by a gas-permeable membrane. Occasional activation of the gas generator thus makes it possible to test the sensor function, but it fails to provide information on the state of the transport paths via which the target gas enters during regular measuring operation. These transport paths are determined, for example, by a gas-permeable membrane, through which the target gas can reach the detector electrode.
Furthermore, it is known that test gas can be sent through a membrane, which is also connected to a gas generator and a sensor, in a similar manner (EP 0,744,620 B1). It is difficult to infer the state of the outer membrane granting access for the gas to the detector electrode in this case as well.
Furthermore, it is known that test gas can be injected into a test gas chamber via detector electrodes in the interior of the sensor housing. The test gas chamber is arranged downstream of the outer gas inlet (U.S. Pat. No. 6,635,160). However, the entry of gas from the outside to this chamber and consequently also to the detector electrode of the sensor continues to be untested in this design as well.
Furthermore, diagnostic methods for sensors are known, with which test gas is pressed mechanically through an aperture to a sensor, delivered by adding a propellant or is moved to the sensor by thermal expansion (U.S. Pat. No. 4,151,739). These methods also have the drawback that the path of the ambient gases to the detector electrode is not tested under realistic conditions.